In order to convert the optical signal into the electric signal at a high speed, a photodiode using a photoelectric conversion phenomenon within semiconductor is often used. As the kinds of photodiode, there are a pn type, a pin type, a Schottky type, an MSM (Metal-Semiconductor-Metal) type, or the like. The main factors to limit response speed of photoelectric conversion in the photodiode are circuit time constant determined by a product of electric capacitance made by a depletion layer and a load resistance, and carrier transit time required for a carrier to pass the depletion layer. In order to improve the responsiveness of the photodiode, it is required to reduce the circuit time constant and the carrier transit time. Further, the photodiode is required to be highly efficient and have small dependence on an incident light polarization direction other than high-speed capability.
One of the photodiode capable of a high-speed response is the MSM type. This is expected as a photoelectric conversion element used in the information processing and communication field. This MSM type photodiode is, in general, a kind of the Schottky photodiode that has a Schottky barrier near each of two electrodes. In the MSM type photodiode, by using an electrode which has a comb-like structure, it is possible to apply a high electric field to a light-absorbing layer at a low voltage and reduce the transit time of the carrier, and thereby achieving relatively fast response speed.
On the other hand, since incident light is reflected by an electrode on a light-receiving surface, there has been a problem of reduced quantum efficiency. Moreover, there has been a trade-off relationship in which in order to improve the responsiveness, when the light-absorbing layer is thinned and the carrier transit time is shortened, the efficiency is reduced. In recent years, various attempts have been made to improve the speed and efficiency of the MSM type photodiode than has heretofore been the case using metal surface plasmon.
For example, in a photoelectric coupler disclosed in Patent Literature 1, a device configuration is used in which inter-digitated metal electrodes, which are periodically arranged over a flat semiconductor surface, are disposed so that positive and negative electrodes are nested and face each other. Patent Literature 1 describes an MSM type photodiode that is made by coupling between incident light and surface plasmon by resonance. Further, it is mentioned that a diffracted wave generated by the metal electrode is coupled to a local wave, and is confined in a waveguide. However, the configuration of the metal electrode that efficiently generates the diffracted light by the surface plasmon resonance is not mentioned.
Further, as for the method to couple the diffracted light to the waveguide formed by the light-absorbing layer, only a wavenumber matching condition is written, and the waveguide structure and positional relationship with the metal electrode for improving coupling efficiency is not mentioned. Accordingly, in the MSM type photodiode disclosed in Patent Literature 1, since efficiency for generating the diffracted light of a desired order is low, and the coupling efficiency with the waveguide formed by the light-absorbing layer is low, consequently the quantum efficiency becomes low. In addition, in the MSM type photodiode disclosed by Patent Literature 1, dependence on the incident light polarization direction is large, and efficiency is remarkably reduced to a polarization direction that cannot excite the surface plasmon.
Patent Literature 2 describes an MSM type photodiode with improved efficiency even in the case of a thin light-absorbing layer by providing a Bragg reflection mirror to a lower part of the light-absorbing layer to form a cavity configuration with an electrode as an upper mirror, and confining zero-order transmitted light.
In order to obtain enough efficiency in the MSM type photodiode disclosed in Patent Literature 2, it is necessary to increase Q value of the cavity. However, it is difficult to permit light to enter the cavity with high Q value in the first place, thus the incident light is reflected by the electrode functioning as an upper mirror before entering the cavity. As a result, it is difficult to obtain sufficient quantum efficiency.
Patent Literature 3 mentions that a multiple quantum well layer as a core layer is sandwiched between upper and lower clad layers to form an optical waveguide, and light is absorbed by the multiple quantum well layer. However, it is not mentioned to improve the efficiency of the MSM type photodiode using the diffracted light.